Initial commit: video effects processing system

Add S-expression based video effects pipeline with modular effect
definitions, constructs, and recipe files.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

recipe-sexp-slice.sexp

@@ -0,0 +1,30 @@
+; Test recipe using S-expression slice-on construct
+(recipe "sexp-slice-test"
+  :version "1.0"
+  :encoding (:codec "libx264" :crf 22 :preset "fast" :audio-codec "aac" :fps 30)
+
+  (analyzer beats :path "../artdag-analyzers/beats/analyzer.py")
+
+  ;; Import slice-on as a user-defined construct (not the built-in!)
+  (construct slice-on :path "constructs/slice-on.sexp")
+
+  (def video-a (source :path "monday.webm" :description "Video A"))
+  (def video-b (source :path "new.webm" :description "Video B"))
+
+  (def audio (-> (source :path "dizzy.mp3" :description "Audio input")
+                 (segment :start 0 :duration 10)))
+
+  (def beats-data (-> audio (analyze beats)))
+
+  ;; Use the S-expression slice-on with a reducer function
+  (def segments (slice-on beats-data
+                  :init 0
+                  :fn (fn [acc i start end]
+                        {:source (nth (list video-a video-b) (mod acc 2))
+                         :effects (list)
+                         :acc (inc acc)})))
+
+  (def synced-video (-> segments
+                        (sequence :resize-mode :fit :priority :width)))
+
+  (mux synced-video audio))